The present invention generally relates to the detection of insulation faults in electrical wiring that may lead to electrical arcing. More specifically, it relates to a system of diagnostic tools by which insulation faults in wiring systems can be revealed and located before they develop into arcing faults. Although principally directed towards the diagnosis of wiring faults in aircraft wiring systems, these methods and tools apply directly to other fields including residential, industrial and commercial power systems. In this patent the word xe2x80x9cfaultxe2x80x9d is used to refer to both a degradation of the insulation, e.g., an xe2x80x9cinsulation faultxe2x80x9d and a specific failure, e.g., an xe2x80x9carcing fault.xe2x80x9d
A parallel arcing fault occurs when an undesired electrical arc bridges the gap between two conductors or a conductor and ground. Since the dielectric strength of air is known to be approximately 31 kV/cm, it is generally understood that exposed conductors in air and at line voltages (e.g., 117V rms) must come to within a few mils (1 mil=0.001 in) of each other before an arc can strike (Note that 167 Volts peak divided by 31 kV/cm is 2.1 mils). Power distribution systems are therefore commonly designed to avoid this by maintaining conductor separation much greater than a few mils and/or providing adequate insulation between the conductors. It is also understood that parallel faults may develop if the separation between said conductors is inadvertently diminished or if the integrity of the insulation is violated as the result of, for example, chaffing caused by mechanical vibration. In addition to these obvious scenarios there are subtler, less obvious ways in which parallel faults might develop, particularly in the aircraft environment.
First there must be exposed conductors. These can be found at the terminals of circuit breakers, on terminal strips and at some connector terminals. Conductors inside wires may become exposed as a result of aging cracks or holes in the insulation. In October of 2000, an FAA aviation industry task force reported that during the inspection of a relatively small amount of Kapton wiring on both a  greater than 20 year old Airbus A300 and a Lockheed L-1011, 9 cracks that exposed the conductor were found on the former and 13 cracks on the latter. See Christopher Smith, Transport Aircraft Intrusive Inspection Report, The Intrusive Inspection Working Group (Dec. 29, 2000). From this limited data they extrapolated that there might be up to 900 cracks on the A300 and 3,000 on the L-1011. It should be recognized that even a large number of aging cracks in the wire insulation pose little danger of arcing unless the separation between two cracks or a crack and the airframe becomes small enough for an arc to occur at normal operating voltages. How small? Based on a simple linear extrapolation of the dielectric breakdown voltage of air the separation would have to be on the order of several mils or less for arcing to occur. Unfortunately, parallel faults can develop across separations substantially greater than this due to secondary environmental influences.
Once there are exposed conductors within a fraction of an inch of each other or the airframe, initial conduction across the gap can develop in several ways. First, if a voltage surge high enough to span the gap occurs, resulting from an inductive switching transient or perhaps induced by a lightning strike, the localized heating from the momentary arc can carbonize insulating material under the arc, including dust or other contamination on the surface of the wire, and form a high-impedance conduction path. A second and perhaps more likely scenario involves water that normally condenses on the inner shell of the aircraft as the outside temperature drops precipitously during flight. This condensation water readily dissolves impurities that are present and becomes a mildly conductive electrolyte that can conduct a small AC current across the gap. This current produces heat and the heat evaporates the water leaving behind molecular islands of salt that eventually form a kind of archipelago of larger conductive islands. Each time it gets wet, the electrolyte itself will support current flow and add more salts to the developing archipelago. The arc breakdown potential is proportional to the sum of the distances between conductive islands.
What happens next depends on the wire insulation material. Both a Polyimide plastic, sold under the brand name xe2x80x9cKaptonxe2x80x9d, and Fluorocarbon plastic, sold under the brand name xe2x80x9cTeflonxe2x80x9d, have been widely used in aircraft wiring. If the insulation is Teflon, this low-current arc will repeatedly extinguish with little damage done. If the insulation is standard Kapton (i.e., non fire-resistant), the low-current salt bridge arc will likely soon involve the Kapton itself, expand and escalate rapidly into a near explosion of current that often destroys not only the wires involved, but also adjacent ones in the wire bundle.
A number of articles in the press have noted the apparent arcing danger of Kapton insulation. In the presence of a low-current arc Kapton insulation can easily develop into an explosive arc while Teflon exhibits only gradual, slow deterioration of the Teflon heated by the arc. Research by the present inventor has shown that Kapton can become conductive in the near vicinity of an arc. Localized heating of the Kapton apparently oxidizes portions of it to a conductive intermediary (remaining amber in color, it does not appear to be completely oxidized to carbon at this stage) that reduces the arc gap and increases the electric field strength. Under a stereo microscope, one can see minute pieces of Kapton conducting current and arcs jumping from the glowing Kapton to the metal conductor. Insulating materials like Teflon contain halogens that inhibit oxidation by producing by-products that are more electronegative than Oxygen, e.g., Fluorine. Standard Kapton doesn""t have this fire inhibitor and this may, in part, explain the difference.
The formation of aging or stress-induced cracks in the insulation and the repetitive condensation, wetting, and low-current arc induced evaporation cycle together form a progressive degeneration process that can lead to parallel arcing. Mechanical chafing of the wire insulation can also lead to parallel arcing. As the world""s fleet of commercial and private aircraft age, particularly now that many aircraft over 20 years old are still flying, the likelihood of such faults occurring increases. If these developing faults can be detected early enough, the insulation could be repaired or replaced before the fault develops into a dangerous arcing fault. A need exists, therefore, for a means by which wiring harnesses could be tested to reveal these conditions as they are developing.
The present inventor realized that developing parallel faults, due to mechanical chafing or aging cracks in the insulation, for example, will generally exhibit a progressively declining breakdown voltage until a point is reached where the arcing becomes self-sustaining and dangerous. Such developing faults in the insulation are initially non-conductive and usually so small as to make no perceptible change in the characteristic impedance of the cables. The only practical way to reveal reduced conductor spacing (or a non-conducting salt bridge) is to apply a higher-than-normal voltage to the junction, a testing procedure commonly referred to as HiPot (High Potential) testing. A traditional DC HiPot tester, however, which allows 10 ma of current to flow after breakdown, can itself heat the insulator enough to form a carbon track and damage the insulation. A conventional HiPot tester can also damage equipment left connected to the harness during testing. A further need exists, therefore, for a means by which the breakdown voltage can be measured without damaging the wire insulation or any electronic devices inadvertently connected to the harness.
Wire harnesses in modern aircraft are dense, multi-legged, and routed throughout the planexe2x80x94up to 140 miles of wire in a typical wide-body jet. Hundreds of connectors are placed along the harnesses to allow modular assembly and disassembly of components. Because access to wiring harnesses is very limited on an operational aircraft, such testing is probably best done during periodic heavy-checks, whereupon panels and floorboards are removed to facilitate access. Even in this case, however, specific wire bundles may be very long and difficult to access. There exists a further need, therefore, to provide practical means to physically locate the developing parallel fault once it has been revealed.
One principal idea of the present invention is based on the understanding that as a parallel fault develops over time, due to chaffing, etc., the dielectric withstanding voltage of the fault will fall in approximate unison. If a voltage higher than the normal operating voltage is applied to such an imminent fault, it can be made to flash over before it would in normal operation thus revealing the fault before it becomes an actual arcing fault.
The dielectric voltage test is performed according to the present invention by charging the interwire capacitance between a first Wire Under Test, or WUT, and the remaining grounded wires of the floating cable harness, using a microampere, high-compliance current source. In this manner, the voltage on the WUT rises from zero in a ramp-like fashion to a specified maximum test voltage. If a fault exists between the WUT and any other wire or ground (the airframe) that has a breakdown voltage less than the maximum test voltage, a single low-energy discharge will occur at the fault, discharging the cable capacitance into the arc. Since the interwire cable capacitance will be on the order of hundreds of picofarads only, the total energy in the discharge will be low, on the order of microjoules. This is on the order of the energy contained in a static spark discharge when walking across a carpet and then touching a grounded surface and is low enough to avoid damaging the wire insulation at the fault. Also, by charging the cable with a microamp-range current source no danger is presented to any devices that may inadvertently be left connected to the harness during the test. If, for example, connectors at the far end of the harness are inadvertently left plugged into their loads, a 1 microamp current source will only be able to charge the line to a few millivolts. In this case, the test system will sense this condition and indicate to the user to disconnect the loads before proceeding.
With a 1 microamp current source, a cable interwire capacitance of 1000 pf, and a maximum test voltage of 1500 volts, for example, the complete ramp sequence for the first WUT will take less than 2 seconds. Assuming no breakdown, an automatic sequencer then switches to the next wire in the harness, makes this the second WUT, grounds the others, and repeats the sequence. In this manner, the entire harness can be quickly and automatically tested for parallel faults between conductors or between any conductor and ground.
If a fault exists with a breakdown voltage below the maximum test voltage, a single micro-energy discharge will occur across the fault. A second principal idea of the present invention is based on the realization that the leading edge of this discharge will be extremely clean and fast, dropping from the high breakdown voltage to zero in a fraction of a nanosecond. Because electromagnetic radiation travels at about 1 ft/ns any method for locating the arc based on the measurement of time delays would have to resolve time differences on the order of a nanosecond or better. To obtain repeatable and, therefore, useful results, the edge of the signal received signal must be very sharp, clean and repeatable to a nanosecond or better. The inherent speed and amplitude of the edge produced by the single spark discharge described above meets this criteria and therefore makes possible several different locating methods, to be discussed later.
The sharpness of the received leading edge depends on how quickly the stored charge can be delivered to the arc. If, for example, a discrete capacitor is connected to the arc gap with a wire, the charge stored in the capacitor must travel through this wire to be delivered to the arc. The inductance of this wire together with whatever capacitance exists at the arc forms a low-pass filter that slows down the leading edge. A controlled-impedance cable such as a coaxial cable, on the other hand, acts like a transmission linexe2x80x94the distributed inductance and capacitance work in unison, transferring the charge back and forth much in the same manner as the mechanism that allows the propagation of electromagnetic waves through space. Thus, the fast leading edge is preserved in a controlled impedance cable. Indeed, it is well known by those skilled in the art that the most convenient means of producing a fast leading edge in the laboratory is to discharge a coaxial cable charged to a high voltage.
A cable harness typical to aircraft wiring is not designed to be a controlled impedance transmission line. Research by the present inventor, however, has shown that because the wires are tightly and neatly bundled according to aircraft harnessing standards, and because all the wires in the bundle are grounded except the single WUT, the impedance becomes relatively constant and the combination acts like a transmission line. Depending on the number of wires and the thickness of the insulation, the resulting impedance is typically 50-80 ohms. Even when individual wires feed off the harness the effect on the impedance at that point is minimal because that wire is only one of many that serve as the ground return.
In accordance with the present invention, this fast edge from the micro-energy arc induced at the fault is used in two different ways to determine the location of the arc. In a first method, based on conducted energy, the difference in arrival times between the reception of the leading edge at one end of the cable harness and a second reflected edge at the same end of the harness, is used to calculate the approximate distance down the cable that the arc originates. In a second spatial method, based on radiated energy, two or more miniature high-speed electromagnetic radiation receivers are employed and the difference in arrival times is used to calculate where in the space between the receivers the arc originates. With one pair of receivers, the arc can be located in one dimension, with two pairs in two dimensions, and with three or more pairs in three dimensions. A third and final arc locating method uses a handheld ultrasonic monitor to measure and indicate the distance from the operator to the arc. It measures both the electromagnetic pulse from the arc and the ultrasonic emission from the arc and uses the difference in arrival times to calculate the distance to the arc.
Though the spatial arc locating method described above is presented as an adjunct to the MED tool, it can be also used separately to rapidly detect and locate in space any type of electrical arcing, including series or parallel arcing. Though perhaps less immune to extraneous noise than other arc detection methods, the spatial method offers the ability to both detect and locate an arc within a microsecond or less. Existing known methods for electrical arc detection are much slower and cannot locate the arc. Thus, the spatial method offers real advantages over existing arc monitoring methods in certain applications, particularly those where there is some control can be exerted over extraneous noise. One application possibility is the use of this spatial arc locating method inside an aircraft during a heavy check procedure as a non-intrusive method to detect and locate electrical arcs as individual systems are turned on and off.
The preferred embodiment of the present invention is a Parallel Fault Diagnostic System that comprises three separate components. The first component, referred to as the Micro-Energy Dielectric Tool or MED tool, is a handheld, battery-operated unit that plugs into a cable harness using an adapter cable, and applies a low-current, high-compliance current source sequentially to each conductor in the harness while grounding all other conductors. The interwire capacitance between each conductor and the others is thus charged in a ramp-like fashion to a specified maximum test voltage. An internal microprocessor system takes various measurements during this ramp process and, from these, calculates the insulation resistance and capacitance for display to the user. If a breakdown occurs between the charged wire and any other wire or ground, the microprocessor will record the voltage level at which the breakdown occurred and switch automatically to a fault-locating mode. High-speed circuits internal to the MED tool will now process the received high-frequency edges and attempt to determine how far down the cable the fault is. If the fault is from wire to wire, the signal will usually give a clear reading of this distance. If the fault is from a wire to ground (the airframe) the signal may be discerned using other techniques described below to calculate the distance.
A second component, referred to here as the Electromagnetic Locating Tool or EML tool, is another battery-powered, handheld unit that provides a second independent method for locating the arc. The EML tool consists of a small, handheld controller and two or more small receiver units, each connected to the controller with a coaxial cable. The receivers are placed at the extremes of the area to be monitored and the controller is held in the technician""s hand. This EML tool works by receiving the radiated electromagnetic edge produced by an arc simultaneously with multiple receivers, and then calculating the location of the arc relative to the controller based on the difference in signal arrival times.
A third component, the Ultrasonic Locating Tool or USL tool, is another handheld unit that provides yet a third method of locating the fault. The USL tool senses both the electromagnetic edge and the ultrasonic radiation from the discharge and, by timing the difference in arrival times, determines the distance from the user to the arc.
The MED tool may work in unison with the EML and USL options.